Method for determining an imbalance of at least one cylinder

Abstract

A method for determining an imbalance of at least one cylinder in an arrangement of at least two cylinders in a system is provided, which includes an internal combustion engine, the imbalance of the at least one cylinder being present in relation to at least one property of an exhaust gas of the internal combustion engine and the determination of the imbalance of the at least one cylinder occurring with a sensor device for detecting at least one property of the exhaust gas of the internal combustion engine. A diagnostic threshold is ascertained through a dynamic characterization of the system with the sensor device, the dynamic characterization taking place after an excitation of the system. The diagnostic threshold makes it possible to delimit a range of potential erroneous detections which are caused due to a dispersion of an evaluating signal resulting from the imbalance of the at least one cylinder.

What is claimed is: 
     
       1. A method for determining an imbalance of at least one cylinder in an arrangement of at least two cylinders in a system having at least one control unit and a sensor device, which includes an internal combustion engine, the method comprising:
 determining the imbalance of the at least one cylinder with the sensor device for detecting at least one property of an exhaust gas of the internal combustion engine, the imbalance of the at least one cylinder being present in relation to at least one property of the exhaust gas of the internal combustion engine, the determining the imbalance comprising:
 applying an abrupt excitation to the system, wherein the abrupt excitation is a sudden change in an injection quantity of fuel into the system that applies a forced amplitude to the internal combustion engine at a time outside of a normal operation of the internal combustion engine, the abrupt excitation resulting in a dynamic characterization of the system; 
 setting a chronological progression of a lambda value based on the forced amplitude such that the determined imbalance of the at least one cylinder is determined outside of a range in which a lambda 1 ripple occurs; and 
 ascertaining a variable diagnostic threshold based on the dynamic characterization of the system with the aid of the sensor device, the variable diagnostic threshold varying as a function of the dynamic characterization; 
 
 determining that an error is present in the system based on the determined imbalance of the at least one cylinder and the ascertained variable diagnostic threshold; and 
 displaying and/or storing the error via an on board diagnostic system. 
 
     
     
       2. The method of  claim 1 , wherein the variable diagnostic threshold is ascertained by comparing at least one parameter of the dynamic characterization with at least one predefined value. 
     
     
       3. The method of  claim 1 , wherein for the purpose of the dynamic characterization of the system, an abrupt response of the system to the abrupt excitation of the system is used, and wherein at least one rise time of the abrupt response to the abrupt excitation and/or a down time between the abrupt excitation and the abrupt response is used as the at least one parameter. 
     
     
       4. The method of  claim 1 , wherein the sensor device detects an oxygen content of the exhaust gas, the oxygen content of the exhaust gas being indicated in the form of the lambda value. 
     
     
       5. The method of  claim 1 , wherein the lambda value, at which the determination of the imbalance of the at least one cylinder takes place, is set unequal to 1.0. 
     
     
       6. The method of  claim 1 , wherein the lambda value, at which the determination of the imbalance of the at least one cylinder takes place, is set to a value of at least 0.9. 
     
     
       7. The method of  claim 1 , wherein a first period, during which the lambda value is set to a first value below 1.0, is followed by a second period, during which the lambda value is set to a second value above 1.0, or a first period, during which the lambda value is set to a first value above 1.0, is followed by a second period, during which the lambda value is set to a second value below 1.0. 
     
     
       8. The method of  claim 1 , wherein the second period is followed by an interval during which the lambda value is set to the value of 1.0, the interval being followed by the first period, the interval having a duration which exceeds the duration of the first period and/or the second period, the duration of the first period and/or of the second period being maximally 3 s. 
     
     
       9. The method of  claim 1 , wherein the sensor device includes at least one filtering device for detecting and/or suppressing at least one spectral component in a frequency spectrum of an evaluating signal resulting from the imbalance of the at least one cylinder. 
     
     
       10. The method of  claim 1 , wherein the variable diagnostic threshold is ascertained by comparing at least one parameter of the dynamic characterization with at least one predefined value, wherein the predefined value is retrieved from at least one of a characteristics map and a characteristics function. 
     
     
       11. The method of  claim 1 , wherein the lambda value, at which the determination of the imbalance of the at least one cylinder takes place, is set to a value of at least 0.95. 
     
     
       12. The method of  claim 1 , wherein the lambda value, at which the determination of the imbalance of the at least one cylinder takes place, is set to a value of maximally 1.1. 
     
     
       13. The method of  claim 1 , wherein the lambda value, at which the determination of the imbalance of the at least one cylinder takes place, is set to a value of maximally 1.05. 
     
     
       14. The method of  claim 1 , wherein the second period is followed by an interval during which the lambda value is set to the value of 1.0, the interval being followed by the first period, the interval having a duration which exceeds the duration of the first period and/or the second period, the duration of the first period and/or of the second period being maximally 1 s. 
     
     
       15. The method of  claim 1 , further comprising displaying the error via the on board diagnostic system.